1. Field of the Invention
The present invention relates generally to optical fiber devices and methods, and in particular to improved techniques for generating cylindrical vector beams.
2. Description of Prior Art
A cylindrical vector beam is a polarized beam having a polarization state with rotational symmetry about the axis of the beam, and includes radially polarized beams and azimuthally polarized beams. Radially polarized beams in particular have recently attracted a significant amount of interest because of their unique properties. The longitudinal electric-field component generated when a radially polarized beam is focused with a high-numerical-aperture systems results in high intensities with a zero Poynting vector along the optical axis. This property enables longitudinal-field spot-sizes smaller than allowed by the diffraction limit, enhanced laser machining, efficient optical tweezers, and tailoring atomic dipole states for quantum information.
Current approaches for generating radially polarized beams using free-space resonators or fibers are fraught with instability problems. One issue that must be overcome in a fiber-based approach is that, within an optical fiber, radially polarized beams co-exist in with three other almost degenerate modes that share the same mode intensity profile but have different polarization-vector orientations. Inadvertent coupling between these modes produces the more familiar, rotationally unstable, first higher-order antisymmetric LP11 mode pattern, rather than the desired radially polarized mode.
The rotationally unstable LP11 mode resulting from the coupling of the four almost degenerate eigenmodes in a multimode fiber is akin to a typical single-mode fiber (SMF), in which the conventional LP01 mode is two-fold degenerate with identical intensity patterns but different polarization orientations. Slight perturbations result in a coupling of these polarizations, thereby rendering polarization-maintaining (PM) operation impossible in an SMF.
The LP11 field pattern not only rotates for even the slightest perturbations in the fiber, its polarization vector also changes orientation. Indeed, previous efforts at generating radially polarized modes with fibers have achieved it either in very short, straight segments, and/or with low modal purity in cavities similar to those employed with bulk laser rods.